The Effects of Temperature and Rate of Dynamic Stressing on the Tensile Strength of Monograde and Multigrade Oils 2006-32-0017

This paper investigates the effects of temperature and the rate of dynamic stressing on the ‘effective’ tensile strength (or ‘cavitation threshold’), F_c, of commercial monograde and multigrade oils. Measurements were conducted by a dynamic stressing technique, involving a pulse of tension (‘negative pressure’) created by the reflection of a pressure pulse at a boundary, over a range temperatures, T, 25°C≤T≤110°C. Two monograde oils (API CC), 10W and 40, and three multigrade oils (two 10w-40 oils; API SF, ACEA B3-98 and API SL/CF, ACEA A3/B3; and a 15W-40 oil; API SL/CF, ACEA A2/B2) were tested. For the monograde oils F_c was found to be in the range 153bar≤F_c≤106bar (25°C≤T≤110°C) for the 10W oil and 185bar≤F_c≤133bar (25°C≤T≤110°C) for the 40 oil. Over the same temperature range, the tensile strengths of the 10W-40 multigrade oils were found to be comparable to the 10W oil F_c at 25°C and tend towards the 40 oil F_c value at 110°C. These values are higher than those previously reported and this is discussed in terms of the higher rate of stressing employed in the current work. It is expected that the higher the rate of dynamic stressing, the higher the value of F_c, and this demonstrated by measurements on the 15W-40 oil (25°C and 100°C) for different rates of stressing (>0.4bar/μs and <0.08bar/μs). These experiments, which are the first reported of their kind on motor oils, show that F_c is approximately 5x larger at the higher stressing rates (175bar compared to 35bar at 25°C, and 135bar compared to 25bar at 100°C). These results indicates that the effects of temperature and the rate of dynamic stressing are important considerations in understanding the cavitation properties and performance of multigrade oils.